I. Field of the Invention
This invention relates to improvements in the Fischer-Tropsch process, and Fischer-Tropsch catalysts. In particular, it relates to improved ruthenium catalysts, and process for using such catalysts in Fischer-Tropsch synthesis to produce hydrocarbons.
II. The Prior Art
Fischer-Tropsch synthesis originated with Franz Fischer and Hans Tropsch in the early nineteen twenties with the recognition that an admixture of carbon monoxide and hydrogen passed over iron turnings at 100-150 atmospheres and about 750.degree.-840.degree. F., produced oxygenated compounds and a small amount of hydrocarbons. At 7 atmospheres, and later at 1 atmosphere, Fischer found that the distribution between oxygenated and hydrocarbon products could be reversed. Fischer-Tropsch synthesis for the synthesis of hydrocarbons from carbon monoxide and hydrogen is now well known in the technical and patent literature. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. An important advance in Fischer-Tropsch catalysts occurred with the use of nickel-thoria on kieselguhr in the early thirties. This catalyst was followed within a year by the corresponding cobalt catalyst, 100 Co: 18 ThO.sub.2 : 100 kieselguhr, parts by weight, and over the next few years by catalysts constituted of 100 Co: 18 ThO.sub.2 : 200 kieselguhr and 100 Co: 5 ThO.sub.2 : 8 MgO: 200 kieselguhr, respectively. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates. Most commercial experience has been based on cobalt and iron catalysts.
The use of ruthenium as a catalyst for the production of high-melting hydrocarbon wax from carbon monoxide and hydrogen has been known since the late thirties or early forties. Ruthenium is known as one of the more active catalysts, and its selectivity for making methane in the production of hydrocarbons is relatively low. Moreover, it is recognized as having a low carbon dioxide selectivity. The ruthenium catalyst thus behaves somewhat more ideally than many other catalysts, e.g. iron catalysts, in that more of the hydrogen and carbon monoxide of a synthesis gas are converted to hydrocarbons and water in accordance with the idealized equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O; with less of the synthesis gas being converted to carbon dioxide, as in the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2. The low carbon dioxide selectivity makes use of a ruthenium catalyst for the production of hydrocarbons particularly advantageous for use in processing synthesis gas derived by the conventional technique of steam reforming light hydrocarbon gases, e.g. refinery gas and natural gas.
U.S. Pat. No. 4,199,522, which issued on Apr. 22, 1980, to Murchinson et al, discloses an improved Fischer-Tropsch process, and catalyst which, inter alia, (1) consists essentially of about 1-95 wt. % (preferably about 10-50 wt. %) of at least one material selected from the group consisting of the sulfide, oxide or metal of Mo, W, Re, Ru, Ni, Pd, Rh, Os, Ir and Pt; (2) about 0.05-50 wt. % (preferably about 1-10 wt. %) of at least one material selected from the group consisting of the hydroxide, oxide or salt of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba and Th; and (3) optionally a support, e.g. carbon, silica, zirconia, zircon (a mixture of zirconia and silica), titanium dioxide or mixtures thereof. The selectivity of the catalyst to produce C.sub.2 -C.sub.4 olefins, especially C.sub.3 -C.sub.4 olefins, is drastically increased. With the exception of thorium, the "laundry list" of possible first component metals are identical to those exemplified by Murchinson et al as metals previously known to be useful in Fischer-Tropsch reactions to produce a "variety of compounds; both hydrocarbons and oxygenated compounds.
In U.S. Pat. No. 4,042,614 to Vannice et al which issued Aug. 16, 1977, there is disclosed a ruthenium catalyst, the ruthenium being dispersed on TiO.sub.2, other titanium-containing oxides or mixtures of titanium oxides, which provides superior synthesis characteristics in the conversion of carbon monoxide and hydrogen to hydrocarbons, notably olefinic hydrocarbons, particularly C.sub.2 to C.sub.10 olefins. These catalysts, like other ruthenium catalysts, have low methane selectivity, high activity, and low carbon dioxide selectivity. A major disadvantage of this catalyst, one which hampers commercial development, is its low activity maintenance; activity maintenance being defined as the length of time during an operating run that the total carbon monoxide conversion can be kept high, suitably at 90 percent, or higher, and the methane selectivity kept low, suitably at 10 percent, or lower, while maintaining a constant, preferably high, gas hourly space velocity. Activity maintenance is, of course, profoundly important in the consideration of a catalyst for commercial use since, in practice, activity must be maintained by periodically offsetting the effects of deactivation by increasing the operating temperature, and additionally, if desired, by occasionally cutting off the feed to the process, and exposing or contacting the catalyst with hydrogen. The belief is that the hydrogen treatments help to minimize the deactivation rate thereby extending the catalyst life. The elevation of temperature, or application of both of these techniques, has not adequately extended the life of these catalysts. With each temperature increase, there is degradation in product selectivity. Methane selectivity increases and the total yield of liquid hydrocarbon product decreases. Eventually, product selectivity deteriorates to the point where the catalyst must be regenerated, or replaced.